Tracking, i.e. the real-time determination of the spatial relation between objects, is employed in various fields. These include augmented reality, where virtual objects need to be precisely placed with respect to real objects; or navigation, where guidance can be provided between the current and desired location of instruments, or where regions of desired movement may be monitored. For automatic serving of, e.g., camera systems, the location of the moving region of interest may be tracked as well.
In medical applications, a growing field of interest concerns the tracking of the position of objects of various kinds in a human body. These objects to be tracked may, e.g., include devices like cameras, catheters, ultrasound probes, etc., or instruments and tools for the application of substances like medicine or radioactive markers, or surgical instruments for minimally invasive surgery. Currently, there are many different tracking technologies. However, most of them are not suitable for tracking sensors moving within anatomy with no direct access or line of sight, i.e. sensors that are not visible from outside the patient and which are not reachable by a manipulator arm, such as a flexible endoscope or catheter used in various diagnostics or treatment procedures.
U.S. Pat. No. 6,288,785 B1 discloses a system for determining spatial position and/or orientation of one or more objects, the system including an optical subsystem and a non-optical subsystem. US 2007/0225595 A1 discloses a hybrid surgical navigation system for tracking the position of body tissue, including a marker mounted subcutaneously to the tissue and a tracker located above skin level.
A well-known technique for the tracking of devices in a human body is the electromagnetic tracking principle. A typical electromagnetic (EM) tracking system uses a three-axis magnetic dipole source, which is henceforth also called field generator, and a three-axis magnetic sensor, henceforth also called a receiver. From a source excitation pattern of three sequential excitation vectors, each linearly independent of the other two, three sensor output vectors are measured, which contain sufficient information to determine both the position and orientation of the sensor relative to the source, which is equivalent to 6 degrees of freedom (DOF).
Thereby, the electromagnetic field generated by the field generator spreads over a relatively large area, in between the borders of which the receiver can be detected respectively tracked with a certain precision. Generally, the field is not homogeneous, but has gradients, which is typically used in the process of localizing the position of the sensor in the field. Generally, both DC based and AC based systems are used, implying static electromagnetic fields or dynamic electromagnetic fields.
Though in recent years, significant progress has been made in improving the above techniques, there are a number of inherent drawbacks which can hardly, or only with significant effort, be overcome. Mainly, anything in the electromagnetic field which causes the field to be distorted will result in measurement noise and/or errors. For example, ferromagnetic metallic objects cause the field to be distorted, which is particularly relevant in clinical environments.
Further, eddy currents are induced in nearby metals by a changing EM field, which happens throughout each measurement period with AC trackers. DC-based trackers were developed in an attempt to alleviate this problem. The magnetic field produced by ferromagnetic materials affects both AC and DC trackers, and has a frequency dependency which generally diminishes with frequency.
In general, static distortions of the EM field may be determined in a calibration procedure. For example, the distortion may be calculated with respect to some ground truth, e.g. obtained with an alternative tracking system like IR-optical tracking. During operation, this information can then be used to estimate the true position and orientation via different methods. Such a calibration usually can not span the full 6 DOF parameter space, which is due to an exponential increase of data points.
On the other hand, dynamic distortions of the EM field, e.g. due to objects introduced into the field during operation, may be detected with consistency checks between multiple sensors fixed relative to each other. The measured spatial relation between those sensors should be static, and otherwise a distortion of the EM field may be assumed, which is however not a safe conclusion.
To sum up, in contrast to mechanical arms or optical tracking camera systems, known magnetic tracking solutions do not provide the accuracy and stability required by many clinical procedures, while nevertheless, they require significant effort, labor and cost for the correction of errors inherent to the principle, as was described above.
In view of the above, there is a need for a tracking system for tracking elements in a body which overcomes the shortcomings of the known solutions.